The invention relates to indicating ink compositions that change color, preferably to black or dark brown, under sterilization conditions of high temperature and humidity effected during steam sterilization.
In hospitals, clinics and the like, it is standard practice to sterilize various products such as gowns and drapes and various medical devices such as surgical instruments, prior to use by placing them in a steam sterilizer. Where such products or instruments are not pre-packaged in a sterile state it is necessary for the hospital or clinic to sterilize them before use. Furthermore, where these products or instruments are not disposable, and are employed more than once, it is necessary that they be cleaned and otherwise prepared for subsequent use. Prior to such use, however, it is essential that such products or instruments be sterilized. This practice is necessary to avoid infection and prevent contamination from the use of such articles. It is particularly important where the articles have previously been used in the care of other patients. As there is no visual way of determining whether a particular article is sterile or not it has been the practice to use, with the article, when placed in the sterilizer, a color change indicator (e.g., a steam sterilization indicator) that changes color under the steam sterilizing conditions, thus indicating that the particular article or package has been processed through the steam sterilizing cycle.
The indicator may be in the form of a strip, card, or tape, for example, to which an indicating ink composition has been applied. It is generally the practice in sterilizing such articles to gather several articles together and bundle them in a porous wrap held together by pressure-sensitive adhesive tape. The bundle is then placed in a sterilizer with a sterilizing indicator either inserted into or applied to the bundle. Where pressure-sensitive adhesive tapes are used for this purpose, it is convenient to have the sterilization indicator on the tape backing. By observation of the color of the sterilization indicator, one can readily determine whether or not the package has been passed through the sterilization cycle. Steam sensitive compositions and steam sterilization indicator compositions that are generally used for such purposes include a polyvalent metal compound, such as lead carbonate, and sulfur. Color development for such compositions usually involves a change to brown or black. In fact, many conventional steam sterilization indicator compositions that change color in steam sterilization contain lead compounds. Materials printed with such compositions containing lead compounds are increasingly unsuitable due to environmental concerns.
Lead compounds are being replaced by other polyvalent metals such as bismuth. For example, U.S. Pat. No. 3,471,422 (Edbohm et al.) describes a time-temperature responsive color changing indicator composition based on a compound of a polyvalent metal and a sulfur material. The polyvalent metal is lead, copper, cobalt, nickel, bismuth, or cadmium. Also, Japanese Patent Kokai No. 41992!-364,174 (Takemura et al.) and Japanese Patent Kokai No. 41992!-62,746 (Koybayashi et al.) describe bismuth compounds that can be combined with sulfur or sulfur compounds such as thioureas to produce indicator compositions for steam sterilization. However, such compositions do not reliably turn black, or even dark brown, and/or they take too long to turn black or dark brown under sterilization conditions of high temperature and humidity, or when exposed to dry heat they undergo a color change that is not markedly different from the color formed when the same composition is exposed to steam. Thus, there is a need for steam sterilization indicator compositions that do not contain lead, but are reliable indicators.